White light emitting diode (LED) circuits are commonly used in many applications because of their numerous advantages. For example, LEDs have a longer life span than other types of circuits. LEDs are also constructed of environmentally friendly materials. LEDs also have faster “turn on” times and faster “turn off” times than other types of circuits.
There are two prior art methods for adjusting the perceived brightness of LEDs. The first method is to change the magnitude of the LED driving current itself. This method, however, changes not only the perceived brightness of the LEDs but also changes the perceived color of the LEDs. The change in color is referred to as a “color shift.” In many cases it is desirable to avoid the occurrence of the color shift phenomenon. When color shift is undesirable, the second method for adjusting the perceived brightness of LEDs is used. The second method does not change the magnitude of the LED driving current but keeps the magnitude of the LED driving current constant.
Therefore, constant current source circuits are commonly used in LED driver applications. One of the commonly used methods for providing a constant current source circuit utilizes a feedback loop. The feedback loop method uses a current sense resistor that is connected in series with a plurality of LED circuits in order to obtain a feedback voltage FB from the LED driving current. The feedback voltage FB is then provided to LED driver circuit through a feedback signal line. The LED driver circuit uses the feedback voltage FB to regulate the LED driving current.
For example, FIG. 1 illustrates a schematic diagram of a prior art circuit 100 for generating an output current for a plurality of light emitting diodes. An LED driver 110 is employed to provide the output current for the light emitting diodes 120. As shown in FIG. 1, the light emitting diodes 120 are connected in series. The first light emitting diode (LED) is designated with reference numeral 120a, the second LED is designated with reference numeral 120b, and so on. The last LED is designated with reference numeral 120n. The output current I(LED) that passes through the LEDS 120 also passes to ground through a sense resistor 140 (designated R1) as shown in FIG. 1. An output capacitor 150 (designated C1) has a first end connected to the VOUT terminal of LED driver 110 and has a second end connected to ground.
A voltage source 130 is connected to an ON terminal of the LED driver 110 as shown in FIG. 1. The LED driver 110 provides an output voltage VOUT to the LEDs 120 at the VOUT terminal. A feedback node FB is located between the last LED 120n and the sense resistor 140. A feedback signal from the feedback node FB is provided to the LED driver 110 to enable the LED driver 110 to regulate the value of the output voltage VOUT.
The voltage source 130 provides a pulse width modulated (PWM) input voltage signal to the LED driver 110 at the ON terminal. The duty cycle of the output current I(LED) is controlled by turning the LED driver 110 on and off.
Adjustment in the perceived brightness levels of the LEDs is made by adjusting the width of the pulses. A larger duty cycle for the output current I(LED) (i.e., wider “on” pulses) creates a higher level of perceived brightness. A smaller duty cycle for the output current I(LED) (i.e., narrower “on” pulses) creates a lower level of perceived brightness. This technique is referred to as “pulse width modulation (PWM) dimming.”
The prior art circuit 100 shown in FIG. 1 provides PWM dimming for the LEDs 120. However, the rise times (and fall times) of the LED driving current I(LED) are affected by the operation of the output capacitor 150. The output capacitor 150 holds up the output voltage VOUT for a time even though the LED driver 110 is off. This means that the LED driving current I(LED) still flows until the output capacitor 150 is discharged.
This feature is illustrated in FIG. 2. The voltage source 130 is alternately turned on and off at node ON of the LED drive 110. This is shown in FIG. 2A. When the voltage at the ON node is high, then the output voltage VOUT is at its high value and the LED driving current I(LED) is at its high value. When the voltage at the ON node goes to zero, the output voltage at the VOUT node of the LED driver 110 starts to decrease. This is shown in FIG. 2B. At the same time the LED driving current I(LED) also starts to gradually decrease. This is shown in FIG. 2C. The gradual decrease in LED driving current I(LED) is shown designated with reference numeral 210 in FIG. 2C.
When the voltage at the ON node resumes its high voltage value on the next cycle, the operation of the output capacitor 150 causes it to take some time to recharge the output voltage to its maximum level. This is also shown in FIG. 2B. It also takes some time for the value of the LED driving current I(LED) to gradually increase back to its maximum value. This feature is also shown in FIG. 2C. The gradual increase in the LED driving current I(LED) is shown designated with reference numeral 220 in FIG. 2C.
For these reasons it is not possible to obtain a high pulse width modulated (PWM) dimming frequency using a prior art LED driver apparatus of the type shown in FIG. 1.
To solve the problems inherent in the prior art device shown in FIG. 1, other types of prior art LED drivers have been tried. FIG. 3 illustrates a schematic diagram of a prior art circuit 300 for generating an output current for a plurality of light emitting diodes.
An LED driver 310 is employed to provide the output current for the light emitting diodes 320. As shown in FIG. 3, the light emitting diodes 320 are connected in series. The first light emitting diode (LED) is designated with reference numeral 320a, the second LED is designated with reference numeral 320b, and so on. The last LED is designated with reference numeral 320n. The output current I(LED) that passes through the LEDs 320 also passes to ground through a sense resistor 340 (designated R3) as shown in FIG. 3. An output capacitor 350 (designated C3) has a first end connected to the VOUT terminal of LED driver 310 and has a second end connected to ground.
The LED driver 310 provides an output voltage VOUT to the LEDs 320 at the VOUT terminal. A switch 360 is coupled between the VOUT terminal of the LED driver 310 and the first LED 320a. A feedback node FB is located between the last LED 320n and the sense resistor 340. A feedback signal from the feedback node FB is provided to the LED driver 310 at the feedback terminal FB to enable the LED driver 310 to regulate the value of the output voltage VOUT. A Zener diode 370 is connected between the VOUT terminal of the LED driver 310 and the feedback node FB as shown in FIG. 3.
Prior art circuit 300 also comprises a voltage source 330 that has a first end connected to the switch 360 and that has a second end connected to ground as shown in FIG. 3. The output of the voltage source 330 is designated with the letters LED_ON. Prior art circuit 300 operates by opening and closing switch 360 to connect the voltage source 330 to the LEDs 320. This technique is able to shut off the LED driving current I(LED) in a very short time. However, this technique also breaks the feedback loop.
Zero feedback voltage causes the value of the output voltage VOUT to rise higher and higher. Furthermore, when the LED_ON output of the voltage source 330 is reconnected, the residual high values of the VOUT voltage causes the LED driving current I(LED) to overshoot. This stresses the LEDs 320 at the beginning of every ON cycle.
These features are illustrated in FIG. 4. The voltage source 330 is alternately connected through switch 360 to the LEDs 320. The result is shown in FIG. 4A. When the voltage LED_ON is high (“ON”), then (1) the output voltage VOUT is at its high value, and (2) the feedback voltage FB is at its high value, and (3) the LED driving current I(LED) is at its high value. When the LED_ON voltage goes to zero (“OFF”), then the feedback loop is broken and the feedback voltage also goes to zero. This is shown in FIG. 4B.
When the LED_ON voltage goes to zero (“OFF”), then the output voltage VOUT starts to increase. This is shown in FIG. 4C. At the same time the LED driving current I(LED) also goes to zero. This is shown in FIG. 4D.
When the LED_ON output of the voltage source 330 is reconnected, the residual high values of the VOUT voltage causes the LED driving current I(LED) to overshoot. The overshoot in the LED driving current I(LED) is shown designated with reference numeral 410 in FIG. 4D. In addition, it requires some time for the LED driving current I(LED) to settle back down from the overshoot. For these reasons it is not possible to obtain a high pulse width modulated (PWM) dimming frequency using a prior art LED driver apparatus of the type shown in FIG. 3.
Furthermore, a slow PWM dimming frequency can sometimes cause noise problems. This is due to the fact that the output voltage VOUT needs to be charged and discharged in every “on”/“off” cycle. Rapid VOUT change generates an “in rush” current from the input and causes noise from the capacitor 350. In order for the noise to be effectively reduced, the PWM dimming frequency must be higher than an audible range of frequencies.
Therefore, there is a need in the art for a system and method that is capable of maintaining an output voltage for a constant current source circuit. There is a need in the art for a system and method that is capable of regulating an output voltage of a constant current source circuit to a substantially constant value when a feedback loop of the constant current source circuit is disconnected.
An advantageous embodiment of the system and method of the present invention maintains an output voltage of a constant current source circuit. A constant current source circuit is provided that comprises a voltage regulator, a first feedback loop and a second feedback loop that are connected to the voltage regulator, and a sample and hold circuit that is connected to the second feedback loop. The voltage regulator regulates an output voltage VOUT to a reference voltage VREF using a first feedback voltage signal FB on the first feedback loop. The sample and hold circuit samples and holds a second feedback voltage signal VFB from the second feedback loop while the first feedback loop is connected. The voltage regulator regulates an output voltage VOUT to the second feedback reference voltage signal VFB when the first feedback loop is disconnected.
Before undertaking the Detailed Description of the Invention below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.
Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as to future uses, of such defined words and phrases.